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Old 02-20-2013, 08:06 PM   #1
kellymac530 OP
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Muffler Tech?

So I am building a BMW based dual sport and I want to make my own muffler. I have the ability and equipment like pipe benders and welders at my avail but I am not up on exactly what makes the muffler lower the noise out put.

I mean I get the resonance of a larger chamber to some extent and I get that baffling changes and disrupts the sound waves. The principles in general I get.

What I have is the stock BMW RT head pipes. Those collect 2 into 1 and have a 2" {ish} end right near the right footpeg.

I will bend an "S" to cut under the swingarm where the Cat would be and the upwards on an angle and have some kind of inline muffler.

I do mind a bit throatier sound than the stock RT muffler but I do not want it loud like an MX bike. I am thinking of trying an automotive Glasspack like a Cherrybomb just to see. But what I am actually thinking of trying is something like this:

http://www.dynomax.com/mufflers.php?muffler=superturbo

I am thinking stand it on edge with the low end as the inlet and the high end as the outlet. That would keep it only 3" thick which helps with my custom luggage clearance and width AND puts the outlet up out of most water when forging streams.

My issue is that I know ever engine has its own sound wave and the Boxer motor is very different than most. Do you guys n gals think this exhaust will muffle the sound or not? I mean they use that muffler on cars as stock replacements with minimal volume increase.

BUT- I had a loud older generator years ago and I stuck a car muffler on it and welded up a flange and it was still almost the same dB level at least to my ears. It had to do with the thumping of the single.

I have also thought about just getting 2 dirtbike style mufflers and stacking them after RE-splitting the single pipe after the "S" bend.

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Old 02-20-2013, 10:09 PM   #2
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Don't listen to me as most of my attempts have been duh to total failure.

But,

First, I think it is easier to muffle a 6 or 8 cylinder than a single cylinder motorcycle. There is a lot of mixing and cancellation between the cylinders. The single VS multicylinder effect might be what you were seeing on your generator.

Mufflers are not that frequency dependent because the frequencies vary so much with RPM. I don't think the brand of motorcycles matters, but RPM range and number of cylinders counts.

I understand most of the serious calculations done now are with "Computational Fluid Dynamics" using Finite Element Analysis. Not in everybody's toolbox.

Here is an explanation on how mufflers work and some generalized data.
http://auto.howstuffworks.com/muffler1.htm

Sorry I could not help.

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Old 02-20-2013, 11:02 PM   #3
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I'm going to make a muffler for my project. I've got some ideas, I'll let you know if they work.


Oh, and if you'll pardon the spam for a local business here's a source for premade parts:
http://www.mandrel-bends.com/catalog/
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Old 02-21-2013, 03:03 AM   #4
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I have made mufflers, dynoed before and after with no difference at all in the whole powerband. Hotdogs (cherrybomb?) are noisy.

Many years ago we had a long track speedway meeting at a trotting track in the middle of a city, so some big noise restrictions. They made all the bikes have a muffler similar to the ones in your link....I think the exhaust crowd were one of the sponsors. They seemed to fit on the bikes no problem, almost quiet for a speedway bike, but still sounded good. Riders said there was no difference in power.
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Old 02-21-2013, 06:25 AM   #5
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I've done three custom exhausts.

Just buy the muffler from ebay and weld to it. I usually try and find one from a reasonably similar bike.

I put ducat i vance and hines cans on my 950

I put a single ducati arrow can on my fz1

I put a single ducati arrow and the cbr

Check out cone engineering too
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Old 02-21-2013, 08:53 AM   #6
kellymac530 OP
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Thanks for the input, good food for thought.

I knowbrand of bike is not really important, but a boxer motor definately has its own frequency of sound waves, as you state from a thumper to an 8 cyl making such diff waves. The boxer being 180* opposed and odd firing it may be ver different to muffle than say a HD or a Triumph paralell twin.

Thanks for the tech link, I will use some of that info in my thinking process.

As for bends, thanks for that link as well. My best bud owns a muffler shop here in town and I have all access to his bender, I can bend a simple S pretty easily I think. I built my entire exhaust for my Studebaker truck using his shop...free, and cool. That is why I was thinking of a simple cherrybomb style, I have straight dual exhaust, no Y or H connection on my 289 V8 {Stude motor NOT a ford} with 2 small GP mufflers and it is actually very quiet, almost TOO quiet for a Hot Rod truck.
$20 for a GP, could be worth a quick try. I have run the bike with NO muffler before, straight out the collector and I was shocked at how quiet it was, it was loud, but not GSXR 1000 with a race Yosh muffler loud.

Maybe if what ever I come up with I can also always drop some volume with a stack of Super Trapp discs on the end...sparky AND some noise and tuning ability. Keep the ideas coming, and thanks a ton folks.
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Old 02-21-2013, 09:19 AM   #7
DustyOne
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try 1 of those stock ktm cans for the 450-530 size range.

ideally, each cylinder would have 1 of these.

take a look at the flowmaster website, they show how the baffles were designed within their 40 and 50 series. it is just not placement, there is a lot of research going into muffler design.

the exhaust you have chosen to start with will gradually over time get louder and louder as the packing blows out. then at that point out will need to throw it away cuz they are not rebuild able. but they are cheap, and we know it is just a starting point towards your final result

the OEM car companies put tons of money into their truck and performance car exhaust system design trying to satisfy these requirements:
EPA
performance
cost
the accounting department
and do not forget 'sound'

those newer trucks sure do sound way better than the ones made 15 years ago, right off the floor

good luck Kelly
beemer

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Old 02-21-2013, 09:38 AM   #8
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I don't think the muffler makes that much difference especially with your motor not being some fire breather.

I've repacked my arrow mufflers before it's easy. Drill out the rivets, and it comes apart. Buy new muffler packing and rivet it back up. Can't even tell I was in there.

You can buy carbon cans on ebay all the time. I like buying ducati cans because they are bling. Usually they are sold as a set and I try and find ones that are crashed in one side because most ducati owners don't want one wrecked can. So they usually go for cheap. I've bought ones from a 916, st4 & monster.

Personally I would not do a cherry bomb because they are probably heavy.
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Old 02-21-2013, 01:36 PM   #9
Roadracer_Al
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There are a number of factors that influence the "apparent" sound of a vehicle.

Much of the signature sound comes from mechanical noise and intake noise. A BMW has shaft drive, which will contribute a mechanical gear whine to the signature sound. Water cooling has a significant damping effect on mechanical noise. Valve clearances, cam chains, straight cut vs. helical cut primary gears, wet vs. dry clutches. Thin vs. thick castings. Even tire hiss and aerodynamic turbulence make noise. All contribute to the apparent sound signature.

Regarding actual exhaust note, stuff like cam profile and header tube diameter, post-collector exhaust tubing tapers, and open area of the perforated muffler core play a large part in establishing the tone.

In my experience, a large diameter, straight-through core muffler (i.e. a cherry-bomb style) will be loud, no matter what you do..... except making it longer. The more absorption area, the quieter it will be. Motorcycles don't have enough length to get a truly quiet straight-through core muffler. Too-large diameter muffler cores do very little to quiet the exhaust note -- that's why the generator mentioned above didn't get any quieter.

At the heart of making a well-flowing but quiet muffler is the understanding that gas flow and sound energy behave differently. Gas flow actually improves over a large-radius sweeping bend compared to a straight section of pipe. However, sound energy must travel in a straight line until it hits something, where it is either reflected into a new straight line path, or absorbed (and some energy is absorbed even when it's reflected).

OEM mufflers are sometimes restrictive for two reasons - first, they use small diameter cores, and second, they use chambers which cause the gas to make tight U-turns. They do take a lot of sound energy out, however.

The trick, IMHO, is to design your muffler core with some bends so you can't see through the inlet to outlet, a diameter about the same as the collector diameter, use a perforated material with a large percentage of open area, and a sufficiently small perf so the packing doesn't blow out. To combat packing blow-out, I've wrapped cores with 2 turns of copper window screen and tied it all up with stainless safety wire. This really improves the lifespan of the packing.

Hope that you find that helpful.
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Old 02-21-2013, 05:06 PM   #10
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Awesome help folks. Thanks for keeping up the input.

RR Al, I just want to make sure I am getting what you are saying, The muffler does does not need the full U-turn/U-turn like the Turbo muff above, but just a kink or two to break up direct flow?
Then on the core topic, I get that smaller holes in the core will extend packing life, but wouldn't making the holes or screen openings TOO small cut down on muffling because at some point it would almost be like a straight tube?

If I hand make a muffler, when I bend some kinks in it so there is no straight through shot, should I use a perforated tube on the bends, or make a solid corner at the bend/kinks and only have peforated core on the straight shots?

Just so you all know, I would not likely use a Turbo or Cherrybomb as a permanent muffler specifically because of the weight, but if the consenous is that something like the Turbo muff would work, then I would try one and if it sounds ok I might copy the design but with lighter weight material and make it repackable.

This stuff is really helpful.
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Old 02-21-2013, 06:01 PM   #11
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If your interested in cancelling out noise depending on engine frequency do some reading on helmholtz resonators.
Lots off baffled car mufflers are designed using the principle.
But like you already pointed out a baffled muffler is always going to be heavier than a streight through packed style muffler plus the baffled mufflers hold the heat a lot more.
Straight through packed muffler can be verry quite but they need be large.
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Old 02-21-2013, 06:27 PM   #12
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Thanks Joe, I will read more on that, but I just read 4 different pages on the concept, simplified versions of course. As I read the web pages I could not help but think of a 2 stroke expansion chamber and how they use size, shape, length and various cones to bounce the waves back and forth to shape its power curve. Cool stuff. I do not see a 24" ball on the side of this behemoth as a positive though...Fat enough as it is
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Old 02-22-2013, 11:45 AM   #13
Roadracer_Al
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Yes, the hole size & spacing on the perf makes a big difference on the tone of the muffler, as well as the dB reduction.

The real question is "Percent Open Area" -- i.e. how much of the surface area is solid vs. how much is a hole. A higher open area is superior for sound energy attenuation. I've used McNichols to supply perforated stainless sheet, and this is one of the characteristics which they use to describe the hole pattern.

http://www.mcnichols.com/products/perforated/

I've used a 3/16" hole on a 1/4" spacing, which yields a 51% open area. This works pretty well, although it's a little big for best packing retention. So, to indirectly answer your question about the bends, it is mostly the open area that allows sound waves to enter the packing and be dispersed that accounts for the muffling. Having solid tubing in your muffler core goes against your mission of having a quiet muffler.

Regarding the direction of gas flow... basically, the more work you make it do, the quieter the muffler will be. Each change of direction takes energy out. A 180* bend will take out more energy than a 45* bend. Of course, each change of direction will have the result of slowing the gas flow. If that is bad for your particular engine or not can only be determined by testing on a dyno.

Now... the thing to understand is that just making a muffler that has an ideal design won't make any more power than stock.... if the stock exhaust is actually well designed. Older motorcyclists such as myself grew up in an era when you could add 20% power to a motor by swapping exhaust, adding free-flowing filters and rejetting the carbs. Those days are long gone. Most modern bike stock exhausts are pretty damn good. Aftermarket exhausts generally are only good at adding noise and reducing weight.

So, yes, I'd go ahead and install an inexpensive automotive muffler just as an experiment.
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Old 02-22-2013, 03:44 PM   #14
kellymac530 OP
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RRAl
Major thanks.
Power is my least concern, not that I wouldn't mind a bit more power, but as long as my design doesn't rob aby power I am okay with that.

My biggest concerns are:
1} light weight
2} reasonably quiet, but it can be a bit louder than stock
3} as compact as possible so that when I build my panniers I can keep them as narrow, yet roomy as possible.
4} possible performance increase...wouldn't hate that.

That is kinda why I was considering that Dynomax Turbo muffler. It is only 3-1/2" thick, considerably narrower than a stock BMW R1100 muffler. It would also keep the low entry inlet needed on a low headered bike like a boxer but a high outlet, then it seems like it could be pretty good flowing and unrestrictive yet decent silencing.

The big draw back I see to it is weight. Most auto applications could care less if a muffler weighs 3 lbs. or 10 lbs but durability is more important, so thicken it up, heavy rolled seams, thick core...I think you have helped me figure out a plan. Pick up a DynoMax Turbo, tack that on and listen to it, run it around {once my doc clears me to ride again} play with it a bit until I like the performance/sound balance and then hand fab one out of lighter and better materials.

Kind of alot of work I know, but time I have right now and money I do not have alot of, so playing on the welder with some metal is a better option and gives me something to do so I do not go crazy while not working...well, any MORE crazy..

Thanks again folks.

All of this is mute if someone has a scratched up Remus laying around they want to donate to a cool project...
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Old 02-22-2013, 09:50 PM   #15
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Does anybody know the limits of aluminum for mufflers. Maybe you could use aluminum on the outside and steel on the inside.

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